The present invention relates to generators and, more particularly, to apparatus for internally connecting the neutral ends of three stator windings of a large, three-phase generator.
Although the present invention may have applicability to other types of generators, for purposes of concreteness of description, the following disclosure is directed toward a two-pole, three-phase, generator.
A large three-phase generator conventionally includes a laminated stator having longitudinal slots into which conducting bars are installed. The ends of the conducting bars are interconnected by end turns to produce three stator windings whose centers are physically and electrically spaced about 120 degrees apart. Both ends of each winding are conventionally brought out of the generator stator frame to terminals for connection to external circuits.
External connection of the terminals may be arranged to produce either a wye-connected or a delta-connected generator. A wye connection is conveniently formed by placing a short-circuit jumper across the apropriate end of all of the windings. The shorted ends of the windings provides a neutral point at which the windings can be grounded. It is thus conventional practice to identify the three terminals which are to be jumpered together and grounded as the neutral terminals. The remaining three terminals are conventionally called the high-voltage terminals.
Certain test operations such as, for example, periodic high-voltage testing of individual windings, require that provision be made for isolating each phase by removing the jumper connecting the neutral ends of the three windings.
A substantial amount of space is required for mounting the three high-voltage terminals and the three neutral terminals, and for accommodating the internal connections thereto. If the six terminals are not spaced sufficiently apart, the electric fields generated by the current flowing in one terminal may interfere with the current measurements in an adjacent terminal. To satisfy the internal and external space requirements for connection and interference avoidance, it is conventional to provide a large lower frame extension extending below the envelope of the generator stator frame. The three high-voltage and three neutral terminals pass through bushings penetrating the bottom surface of the lower frame extension. Current transformers are disposed on the bushings external to the lower frame extension.
Due to the high currents passing through the high-voltage and neutral terminals, the bottom plate of the lower frame extension, through which the bushings pass, is conventionally made of stainless steel. Such a stainless steel plate is expensive.
The neutral terminals are shorted together in a neutral enclosure external to the generator stator frame. Such a neutral enclosure may be a stainless steel box as large as about four feet by four feet by eight feet. Both the neutral terminals and their associated enclosure are expensive and their elimination is desirable.